Activity of the turbidite levees of the Celtic-Armorican margin (Bay of Biscay) during the last 30,000 years: Imprints of the last European deglaciation and Heinrich eventsarticle scientifique - 2008
Toucanne, Samuel, et al.

High-resolution sedimentological and micropaleontological studies of several deep-sea cores retrieved from the levees of the Celtic and Armorican turbidite systems (Bay of Biscay - North Atlantic Ocean) allow the detection of the major oscillations of the British-Irish Ice Sheet (BIIS) and 'Fleuve Manche' palaeoriver discharges over the last 30,000 years, which were mainly triggered by climate changes.

Between 30 and 20 cal ka, the turbiditic activity on the Celtic-Annorican margin was weak, contrasting with previous stratigraphic models which predicted a substantial increase of sediment supply during low sea-level stands. This low turbidite deposit frequency was most likely the result of a weak activity of the 'Fleuve Manche' palaeoriver and/or of a reduced seaward transfer of sediments from the shelf to the margin. However, two episodes of turbiditic activity increase were detected in the Celtic-Armorican margin, during Heinrich events (HE) 3 and 2. This strengthening of the turbiditic activity was triggered by the meltwater releases from European ice sheets and glaciers favouring the seaward transfer of subglacial material, at least via 'Fleuve Manche' palaeoriver.

At around 20 cal ka, a significant increase of turbidite deposit frequency occurred as a response to the onset of the last deglaciation. The retreat of the European ice sheets and glaciers induced a substantial increase of the 'Fleuve Manche' palaeoriver discharges and seaward transfer of continentally-derived material into the Armorican turbidite system. The intensification of the turbiditic activity on the Celtic system was directly sustained by the widespread transport of subglacial sediments from the British-Irish Ice Sheet (BIIS) to the Celtic Sea via the Irish Sea Basin. A sudden reduction of turbiditic activity in the Armorican system, between ca. 19 and 18.3 cal ka, could have been triggered by the first well known abrupt sea-level rise ('meltwater pulse', at around 19 cal ka) favouring the trapping of sediment in the 'Fleuve Manche' palaeoriver valleys and the decrease of the seaward transfer of continentally-derived material.

The maximum of turbiditic activity strengthening in the Celtic-Armorican margin, between ca. 18.3 and 17 cal ka, was induced by the decay of European ice sheets and glaciers producing the most extreme episode of the 'Fleuve Manche' palaeoriver runoff and a great seaward transfer of subglacial material into the Bay of Biscay, Between ca. 17.5 and 16 cal ka, the turbiditic activity significantly decreased in both Celtic and Armorican turbidite systems in response to a global re-advance of glaciers and ice sheets in Europe. The last episode of ice sheet retreat, between ca. 16 and 14 cal ka, is well expressed in the Celtic system by a new increase of the turbiditic activity. The major episode of sea-level rise at around 14 cal ka ('Meltwater Pulse 1A'), precluding the seaward transfer of sediments, induced the end of turbiditic activity in both the Celtic and the Armorican system.

Although two main phases of global sea-level rise seem to have had an effect on the Celtic-Armorican margin, this work proposes the BUS retreat and associated riverine discharges as the main trigger mechanisms of the turbiditic activity in this region during the last 30,000 years. (c) 2007 Elsevier B.V. All rights reserved.

Sequence stratigraphic models predict increased sediment delivery to deep-water areas during sea-level lowstand. The Armorican margin (Bay of Biscay, western European margin) is an interesting area to test this hypothesis because the margin has a wide continental shelf, still partly flooded during sea-level lowstand, and the Armorican turbidite system has experienced fluctuating sediment fluxes since the last glacial period. The stratigraphic response of the Armorican turbidite system to sea-level oscillations and climate changes was assessed for the last 35,000 years through the study of the Guilcher, Crozon and Audierne levees. Millenial-timescale resolution chronostratigraphy allowed us to reconstruct the sediment accumulation and turbidite frequency, thickness and grain-size over this period of time.

We found that the Armorican turbidite system was sediment-starved during highstand conditions (ca. 8‚Äď0 ka) and that glacial conditions favoured sediment delivery to the deep Bay of Biscay. However, contrary to what would be expected from sequence stratigraphic models, the turbidite flux did not reach a maximum during the LGM lowstand (ca. 26‚Äď20 ka) but at the onset of Termination I (between ca. 20 and 17 ka). This makes the Armorican turbidite system a transgression-dominated one. This sediment pulse can be interpreted as a huge increase in the meltwater discharge of the Fleuve Manche palaeoriver in response to the decay of the British and Fennoscandian ice-sheets. At that time, despite the rising sea-level, a large deltaic system had to have developed on the outer shelf, leading to the delivery of the Fleuve Manche sediment load into the canyon heads. On the other hand, our dataset suggest that the delivery of sediment into canyons was mainly forced by the winnowing and reworking of the sediment stored on the wide, drowned shelf during the last glacial period (between ca. 35 and 20 ka, and between 17 and 8 ka). These findings illustrate the competing influences of accommodation and sediment supply on the Armorican margin over the last 35,000 years, with a shelf acting as a buffer for the sediment supply signal for most of the period, except during the last deglaciation. At that time, the western European sediment-routing system was reactive, the climatic signal rapidly propagated from the southern limb of the European ice-sheet to the Armorican turbidite system. Finally, our study demonstrates that precise reconstruction of turbidite flux in deep-water areas, added to knowledge about the morphology of the margin and the palaeoenvironmental changes (fluvial system, shoreline position, etc.), are crucial for determining the response of turbidite systems to external forcing.

Highlights

? Sediment accumulation, vertical grain-size and turbidite thickness trends from levees of the Armorican turbidite system (western European margin) are examined ? Competing influence of accommodation and sediment supply on the Armorican margin over the last 35,000 years ? The wide continental shelf act as a buffer to sediment supply signal for most of the period, except during the last deglaciation ? The growth of the Armorican turbidite system during the last deglaciation is controlled by the Fleuve Manche palaeoriver and the decaying European ice-sheet

The Fleuve Manche (English Channel) palaeoriver represents the largest river system that drained the European continent during Pleistocene lowstands, particularly during the Elsterian (Marine Isotope Stage 12) and Saalian Drenthe (MIS 6) glaciations when extensive ice advances into the North Sea area forced the central European rivers to flow southwards to the Bay of Biscay (NE Atlantic). Numerous long piston cores and high-resolution acoustic data from the northern Bay of Biscay have been correlated using a well-constrained chronostratigraphical framework, and for the first time allow the estimation of the discharge of the Fleuve Manche palaeoriver at the end of the last glacial period (i.e. Weichselian). Our results, discussed in the light of relationships between river loads and drainage area characteristics, strongly suggest that the Bay of Biscay collected sediment-laden meltwater from the Northern European lowlands and the southern part of the Fennoscandian ice sheet at the end of the last glacial period and particularly during the paroxysmal phase of the European deglaciation ca 20?18 ka ago. This demonstrates that the Fleuve Manche drainage area at that time was equivalent to those dating from the major Elsterian and Saalian Drenthe glaciations. Based on our data, we propose a revision of the palaeogeographical and palaeoglaciological evolution of the North Sea area while showing that the British and Fennoscandian ice sheets entered the North Sea basin until ca 18 ka initiating isostatic adjustment of the basin. This allowed the North Sea fluvial system to flow southwards through the Dover Strait. Finally, we discuss the Fleuve Manche freshwater discharge during the maximum European ice sheet retreat ca 18 ka. The estimated discharge at this time appears important and sufficiently sustained to possibly contribute to the profound destabilisation of the thermohaline circulation recognised in the North Atlantic from ca 18 to 16 ka.

The compilation of results obtained on three giant piston cores from the Whittard, Shamrock and Guilcher turbidite levees reveals a high-resolution stratigraphic record for the Bay of Biscay. Due to the abundance of reworked sediments in these sedimentary environments, a specific methodological approach, based on an X-ray-assisted subsampling phase associated with sedimentological, geochemical and micropalaeontological analyses, was implemented. With an accurate chronological framework, this multi-proxy investigation provides observations on the 'Fleuve Manche' palaeoriver and the British-Irish Ice Sheet (BIS) histories over the last 20,000 years. The results obtained highlight the direct influence of the decay of the BIS on the Bay of Biscay deep-sea clastic sedimentation during the last European deglacial phase. During this period, the annual BIS cycle of meltwater seems enough to generate seasonal turbidity currents associated with exceptional sedimentation rates in all the Celtic and Armorican turbidite systems. With very high sedimentation rates, the turbidite levees represent the main deep-sea clastic depositional area. Long coring combined with a very careful subsampling method can provide continuous high-resolution palaeoenvironmental signals.